Since the discovery of Surface Enhanced Raman Scattering (SERS) on rough silver electrodes, a large volume of work has gone into enhancing this effect with the aim of developing ultra sensitive detection of chemical and biological molecules. Of particular interest are chemical agents used in warfare and biological molecules related to genomic applications and disease agents.
Several approaches to designing SERS substrates based around metallic nanopoparticles and patterned surfaces have been developed. In all of these approaches, the basic guiding physics has been the use of plasmon resonance to enhance local fields along with charge transfer effects which enhance the matrix elements of the Raman process.
Recent work by Lawandy has shown that large field enhancements beyond the conventional effect in non-resonant media such as liquids and transparent solids can take place when the metallic particles (small compared to the wavelengths of interest) are placed in or near amplifying media. In this work, the case of a plasmon resonance resonant with the gain medium response was treated in the Drude Model limit of the metallic particle electronic response. It can also be shown that the use of anisotropic metallic particles such as spheroids and rods of varying aspect ratios can be used to tune the required gain or amplification required to create the large external fields.
Subsequent work has shown that the amplifying medium effect is present in cases of finite particle size and beyond the electrostatic limit of the particle modes. This basic effect of gain and localized plasmon excitations can be further combined with the electromagnetic properties of arrays and photonic band gap structures to provide additional effects on the density of photon states and provide additional enhancements as well as filtering effects useful for the development of chem-bio sensors utilizing amplifying media to create gigantic molecular detection sensitivities.
Further developments have shown that the nanoparticle plasmon resonances on a passive substrate need not be resonant or overlap the gain or absorption medium's resonance. FIGS. 1a and 1b show how the local square of the electric field just outside the particle surface is enhanced in the case of a particle surrounded by a dye film absorbing near and at a frequency far away form the plasmon resonance for the particles on the substrate alone (glass for example) respectfully.
When the film is thick enough (˜0.5 nm), the plasmon response and the external field are driven by the dielectric functions of the dye film and not the substrate. This important fact means that the response takes place at or near the absorbing or amplifying medium's resonances and not the bare plasmon resonance. This in turn means that a number of different metallic particles can be used with virtually any gain or absorption (or both) medium to tune the enhancement to where it is needed for the specific application such as SERS. A large part of this enhancement is due to a factor relating the external field to the internal particle field which is inversely proportional to the highly dispersive absorbing/amplifying medium dielectric functions when the strength of this response is sufficiently strong. Typically this occurs in solid films of high density
FIG. 2 shows a silicon substrate with a random collection of gold nanoparticles prior to the deposition of an absorbing or amplifying film.
The use of a thin (˜0.5 nm-10 nm) film of absorber around a metallic particle results in dramatic enhancements in the field just outside the thin absorber layer. This enhancement is considerably larger than that of the case of a nanoparticle surrounded by a shell of transparent material with no sharp dispersions lines associated with the absorbing transition. FIG. 3 shows the dielectric functions for a solid film of dye coating the particles in FIG. 1. It is clear from these two figures that the enhancement occurs near the absorption resonances and in particular near the dips in the real part of the absorber susceptibility.